It is known that, in a rotary wing aircraft, the acoustic spectra defined in the range between 20 Hz and 20 kHz are the result of the superposition of noises of various origins which can be grouped into two different groups depending on their spectral characteristics, namely pure sounds or narrowband noise and broadband noise.
In the known way, pure sound or narrowband noise occurs in particular, as appropriate:                at the characteristic frequencies of the aircraft driveline;        at the rotational frequencies of the rotor blades (main and tail) and at the harmonics of these frequencies;        at the rotational frequencies of the blades of the compressors of the turbine engine units; and/or        at the rotational frequencies of the blades of the blowers that cool the main gearbox or distribute cabin air and/or of electrical equipment, and at the harmonics of these frequencies,while broadband noise comprises, in particular, as appropriate:        the noise of the boundary layer that develops along the fuselage;        the noise generated by the rotors;        the noise of the air intake and nozzle flows;        the engine noise; and/or        the noise of the circuits that provide the cockpit or the passenger cabin with climate control or heating.        
All this noise is of course troublesome for the pilots and the passengers.
There are various known solutions for reducing such noise inside a rotary wing aircraft, particularly a helicopter.
The object of a first known solution is to reduce the level of vibration or the radiation of sources of noise and/or of the fuselage. To this end, various physical actions can be taken, particularly:                reducing the vibration of the structure and/or of mechanical parts, by damping or modifying the stiffness or the mass;        attenuating the acoustic transmission, by damping or modifying the stiffness or the mass;        a double baffle system, using a space which may or may not be filled with absorbent material, between the radiating structure and soundproofing panels;        acoustic absorption using fibrous or cellular materials; and        acoustic absorption using Helmholtz resonators.        
The first four physical actions listed above make it possible to reduce the overall noise level in a broad range of frequencies, but would lead to a significant and highly disadvantageous increase in mass. In addition, the obtained reduction in noise is not selective enough to eliminate the acoustic annoyance specific to the emergence of pure sounds.
By contrast, the fifth and final physical action listed above makes it possible effectively to reduce narrowband noise, but still only in a narrow band of frequencies, defined during design.
This first solution listed above and based on a passive treatment of the noise is therefore not completely effective, particularly in the case of narrowband noise generated by vibrational excitation.
A second known solution recommends creating passive soundproofing in the form of cladding panels mounted in the cockpit or in the passenger cabin. These panels are designed according to the structural area that is to be treated and according to the spectrum of frequencies to be attenuated.
However, this second solution also has numerous disadvantages and, in particular:                the noise reduction is limited, especially at low frequencies;        the increase in mass is high, and may be several hundreds of kilograms in the case of a large-sized helicopter;        there is a not insignificant loss of volume, especially when using thick panels with a view to improving the acoustic absorption effect; and        there are acoustic leakages, particularly at the wiring holes and the joints between the panels.        
In consequence, neither of these two known solutions listed above is satisfactory in reducing the annoyance caused by noise, particularly narrowband noise.